1. Field of the Invention
This invention relates to the fields of electronics, communications systems and computer systems and more specifically to the new field of local area networks for the passing of data-communications. Specifically this invention relates to an improvement of a technique which controls the exchange of information over a common information channel.
2. Description of the Prior Art
As is well known and understood, because data-communications local area networks are an emerging technology, there are competing systems-design methods using different topology, access, media and protocol schemes. Once the topology of the network--bus or ring--has been decided, one has to consider the procedures for how the network's nodes are to contend equitably for network access. Currently, two dominant network access schemes have proved useable: carrier-sense multiple access with collision detection (CSMA/CD) and collision avoidance or without it (CSMA) and token passing. Both of these schemes do not use a central controller to control access to the medium.
Carrier-sense multiple access (CSMA) alone allows messages to collide. Under a CSMA scheme, all nodes must defer transmission on the medium until the bus channel clears before any one tries to send their messages. On such an event, multiple nodes might attempt to transmit simultaneously; hence one has a high collision rate. To overcome this problem, Collision Detection (CD) is added which requires nodes to wait for varying delay times before retransmitting. However, under many traffic conditions, CSMA/CD incurs higher message collision levels, longer access time delays and reduced throughputs as maximum node-to-node distance increases. It does not suit real time needs because access to the channel is statistical in nature and in addition no priority mechanism exists. Under CSMA/CD, line access is statistical in lieu of deterministic. Moreover, transmission line length must be limited for a given data rate and data size unit if collision detection is to work.
For a token access system, only the node that holds the token at any time can transmit on the network; hence no access control conflict can arise. In the bus networks, the token goes from node-to-node in predetermined fashion, causing this technique to be deterministic and predictable, and not as constrained by transmission line length, frame size or data rate. However, token access has complex algorithms required to set up the station linkages and to recover from network exception conditions (e.g. recovery from failed nodes, recovery from errors) not to mention its complex control methodology.
The master/slave bus control technique, as it is widely known, has an inherent disadvantage in that if the master becomes inoperative for any reason the control of the bus and hence the exchange of all information ceases and thus the connecting bus becomes inactive.
Various attempts to overcome this disadvantage have been proposed in the prior art, some of which however have replaced the efficient central bus control of the true master/slave with some form of less efficient decentralized bus control.
U.S. Pat. No. 4,255,741 contains a scheme for use in loop type data communications systems which is susceptible to a single point failure which can bring the entire system down, if the central controller fails. In U.S. Pat. No. 4,199,661, the invention's primary object is to provide comparison means to each station for the provision of a time slot signal to permit each station to seize use of the channel. All the subscribers need the same polling and comparison circuitry, increasing complexity and costs. In addition, the lack of a central controller makes dynamic alterations in the physical structure complex.
U.S. Pat. No. 4,177,450 requires the unit with the lowest time constant to take control of the bus during each initialization cycle. Therefore, in-time between transmissions on the bus is a function of the unit in control of the bus, or stated in other words, the in-time varies with the time constant of the unit in control of the bus. If an initiation message is not received within the unique time interval of the station having the shortest time interval, then that station transmits an initiation message. Clearly the inter-transmission time interval is a function of the terminal with the shortest interval timer and therefore the utilization of the connecting bus decreases as a terminal with a longer interval timer seizes control of the bus. Hence, the '450 invention is inefficient in comparison to the invention claimed herein, wherein the station which is in control remains in control, and the time interval between transmissions is constant and independent from any channel contention means.